1. Field of the Invention
This invention relates to methods and apparatus for perforating a subterranean well and, more particularly, to techniques for accurately controlling the direction toward which the explosive charges of a perforating gun are aimed when fired in a deviated casing of a subterranean well.
2. Background of the Invention
Bore holes drilled into the earth for the purpose of producing oil and gas often intentionally vary from a vertical axis, utilizing techniques generally referred to as slant hole or directional drilling. Recent advances in directional drilling techniques enable the drilling of nearly horizontal boreholes within the hydrocarbon-bearing formation, and these techniques have become widely accepted in the petroleum industry. Directional drilling is frequently used offshore in order to cover a much larger aerial extent of a downhole reservoir from a reduced number of offshore platforms than is possible utilizing vertical drilling techniques. Another advantage of directional drilling is that the well bore exposes a greater cross-sectional area of the reservoir to drainage, and thus results in higher production rates than vertical drilling techniques.
One particular problem encountered when holes are drilled substantially from the vertical in unconsolidated formations is the production of sands or fines which fall into the wellbore from perforations shot vertically upward through the casing. These fines can gravitate into the casing with the formation fluids and trap the perforating gun, resulting in significant workover costs in order to recover the gun and decreasing the life of wellhead equipment. Accordingly, prior art techniques have sought to control the direction in which the perforating charges are facing when the perforating gun is fired in a deviated section of the bore hole, so that all of the perforations are facing substantially downward and fines in the casing are minimized.
One technique for orienting the direction of fire of perforating guns with respect to a vertical plane has relied upon the placement of ears or lugs on the outside of the gun housing at a position which causes the gun to roll over due to an offset center of gravity, and thereby orient the charges downwardly. Although this technique, which is described more fully in U.S. Pat. No. 4,523,649, has met with limited success, it has not fully overcome the problem. Frictional engagement of the perforating gun with the casing may prevent the gun from rotating the charges to the desired downward direction. Attempts have been made to overcome deficiencies by utilizing swivel joints, as described in U.S. Pat. No. 2,414,997, above the perforating charges. However, even with the inclusion of such swivel joints, guns may still be fired in the upward direction, resulting in the migration of fines into the casing and the potential for the gun becoming stuck in the hole.